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| Name | What Is Semiconductor Doping? |
| Video Uploader | Video From Fiber Optics For Sale Co. |
| Upload Date | This Video Uploaded At 10-10-2016 22:19:01 |
| Video Discription | https://www.fiberoptics4sale.com Semiconductor doping is the process of intentionally adding a small percentage of foreign atoms to extremely pure intrinsic semiconductors to change their electrical properties. Even a tiny percentage doping in single crystal semiconductors makes dramatic effects. Doping is used to increase either the electron or the hole concentration. The common dopants for Silicon are listed in this table. To increase the electron concentration, you can add column V elements such as phosphorus, arsenic, or antimony atoms to the silicon crystal. These dopants are called donors since they are adding more electrons. Phosphorus is most commonly used. To increase the hole concentration, you can add column III elements such as boron, gallium, indium, or aluminum atoms to the silicon crystal. They are called acceptors because they increase holes in the crystal. Boron is the most commonly used. First, let’s use the bonding model to explain how doping works. In the donor doping case, column V elements have 5 valence electrons. When a column V element such as phosphorus is substituted for a Silicon atom in the semiconductor lattice, four of its five valence electrons fit perfectly into the bonding structure, however the fifth electron is extra and doesn’t fit into the structure and is only weakly bound to the phosphorus atom. Since its bond is very weak, at room temperature, this fifth electron gains enough thermal energy and escape from this weak bound. Then it can move freely within the lattice, thus becomes a carrier. This donor doping only increases the electron concentration, not the hole concentration. When the dopant atom loses one electron, it becomes a positive ion and is locked in place by the four covalent bonds. So the release of the fifth electron doesn’t break any atom-to-atom bonds, thus doesn’t produce holes. We use column III elements as the dopants in acceptor doping. Group III elements such as boron have only three valence electrons. When a boron atom is substituted for a silicon atom in the crystal, it can only complete three covalent bonds with its four neighbors, thus leaves one hole, i.e. a missing bond, in the crystal. As we have explained in previous videos, other nearly electrons can be attracted into this hole, and leave behind another missing bond in its original place. Thus this empty hole can be treated as if it can move freely on its own. In the acceptor doping process, only the hole concentration is increased, the electron concentration is not affected at all. The acceptor doping doesn’t release any electrons. The bonding model in the last slide helps us visualize the spatial relationship. However, it does not quantitatively tell us exactly how much energy is involved. Instead, we need to use the energy band model. In donor doping, we said the extra fifth electron is weakly bound around the phosphorus atom and can easily escape at room temperature. However, we need to know exactly the binding energy for this weak bond. The positively charged donor-core plus the fifth electron can be thought as similar to a hydrogen atom (Bohr’s model). However, in a hydrogen atom, the electron moves in a vacuum, in our case, the fifth electron moves through silicon atoms. Thus, we need to use its effective mass instead of electron’s rest mass m0. Also, we need to use the permittivity of Silicon rather than of free space. Then based on the formula for binding energy of a hydrogen electron, we can calculate the binding energy of the fifth electron, and it turns out to be -0.1 electron volt. Here we need to pause and think clearly. If the energy absorbed by the electron is exactly the binding energy, the released electron will have the lowest conduction band energy Ec, as shown in the picture. So that means this fifth electron was at an energy level just |Eb| below the Ec conduction band level, as shown by the dashed line. Let’s remember that the normal band gap for silicon is 1.12 eV, so normal silicon valence electron must gain at least 1.12 eV to become a conduction band electron thus a carrier. But when we dope it with donor atoms, a higher energy level called the donor level Ed is added to the energy band model, which is occupied by the donor’s fifth valence electron. It needs much less, in fact only 0.05 eV, to be excited into the conduction band and becomes a carrier. Here Ed = Ec - |Eb|, Eb is the fifth electron’s binding energy as we calculated in the last page. So the donor doping can increase the semiconductor’s electrical conductivity significantly at room temperature. |
| Category | Science & Technology |
| Tags | semiconductor fundamentals |
